The present invention relates generally to internal combustion engine ignition systems and, more particularly, to a method and apparatus for detecting a low impedance short circuit in an ignition coil secondary winding circuit of an internal combustion engine having a distributorless ignition system (DIS). Distributorless ignition system (DIS) will be used herein to designate an ignition system wherein spark is simultaneously provided to both cylinders of each of one or more pairs of cylinders making up an engine. For such ignition systems, while spark is provided at the correct time for ignition of a fuel charge in one of the cylinders of a pair, spark is also provided in the other one of the cylinders of the pair, which other cylinder is at the end of its exhaust stroke and beginning of its induction stroke.
Stricter vehicle emission standards now required in the United States, Europe and other industrialized countries have created substantial interest in the detection and diagnostics of engine misfiring which is frequently caused by faults in the secondary winding circuit of an ignition coil. Once a misfire has been detected, the fuel control system can be controlled to eliminate or reduce fuel to a malfunctioning cylinder. If fuel continues to be provided to a malfunctioning cylinder, it will result in increased unburned fuel or hydrocarbon (HC) emissions and can lead to damage of a catalytic converter associated with the engine.
Misfiring in conventional ignition systems of internal combustion engines is detected in U.S. Pat. No. 4,918,389 based on a shorter duration of secondary and consequently primary voltage during a misfire. Detected misfires are typically caused by an open circuit in the ignition system due to worn spark plugs, disconnected secondary wiring and the like. A signal indicating the voltage induced in the primary winding of an ignition coil is detected, a reference voltage representing normal firing is generated and the two are compared. The reference voltage is a pulse of predetermined magnitude and duration and the detected voltage on the primary is compared to the pulse to detect if the magnitude of the detected voltage falls below the predetermined magnitude before the end of the pulse. Unfortunately, the disclosed arrangement does not function properly for a DIS.
Open circuit conditions in the secondary winding of a DIS are detected and protected against as described in U.S. Pat. No. 4,969,443. However, the disclosed system does not provide for the detection of a low impedance short circuit in the secondary winding circuit of an engine coil. Such low impedance short circuits are one of the most probable failure modes of an ignition system, for example due to spark plug carbon/ash fouling, and one of the most difficult to accurately detect.
Accordingly, there is a need for reliable and accurate detection of low impedance short circuits in the secondary winding circuits of DIS's to enable an engine controller to adapt fueling in compliance with Federal clean air requirements during such ignition system malfunctions.